BRCA1 is a tumor suppressor gene product dedicated to the suppression of breast and ovarian cancer development. It encodes an 1863 residue nuclear protein, p220. p220 makes contributions to multiple forms of genome integrity control, among which is an ability to support the repair of double strand DNA breaks (DSB) by homologous recombination (HR). Much circumstantial evidence suggests a strong correlation between the ability of p220 to support HR and to perform its tumor suppression function, but how the two are connected, biochemically, is unknown. More specifically, what in vivo biochemical functions p220 must execute to perform its tumor suppressing function and how the two sets of functions are connected to one another are also unknown. In this regard, after the generation of DSBs (e.g. by ionizing radiation), p220 is rapidly attracted to these damaged sites and, some time later, concentrates in focal nuclear structures (aka Ionizing Radiation Induced Foci=IRIF) that form at these locations. What functions p220 performs, once concentrated in IRIF, and how, as is suspected, they contribute to tumor suppression are also unknown. Recently, we and others detected a series of specific biochemical steps that allow p220 to gain access to IRIF. They involve the activities of a nuclear, polyubiquitin-binding protein, Rap80, its associated deubiquitinase, BRCC36, and Abraxas, a nuclear protein that serves as a bridge between p220 and Rap80. All concentrate in IRIF and participate in co-concentrating p220. This proposal is aimed at: a) deciphering the specific biochemical events that tether Rap80 to IRIF; b) at understanding the functional significance associated with p220 concentration in IRIF; and c) learning in what ways these events relate to the execution of p220 cancer suppression function.